linux-kernel - Re: [PATCH 1/2] coda: fix reference counting in coda_file

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Date:   Thu, 22 Apr 2021 09:51:03 -0400
From:   Jan Harkes <jaharkes@...cmu.edu>
To:     Christian König <ckoenig.leichtzumerken@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, coda@...cmu.edu,
        miklos@...redi.hu, akpm@...ux-foundation.org, jgg@...pe.ca
Subject: Re: [PATCH 1/2] coda: fix reference counting in coda_file_mmap error
 path

On Thu, Apr 22, 2021 at 02:39:41PM +0200, Christian König wrote:
> Am 22.04.21 um 14:27 schrieb Jan Harkes:
> > Looks good to me.
> > 
> > I'm also maintaining an out of tree coda module build that people sometimes use, which has workarounds for differences between the various kernel versions.
> > 
> > Do you have a reference to the corresponding mmap_region change? If it is merged already I'll probably be able to find it. Is this mmap_region change expected to be backported to any lts kernels?
> 
> That is the following upstream commit in Linus tree:
> 
> commit 1527f926fd04490f648c42f42b45218a04754f87
> Author: Christian König <christian.koenig@....com>
> Date:   Fri Oct 9 15:08:55 2020 +0200
> 
>     mm: mmap: fix fput in error path v2
> 
> But I don't think we should backport that.
> 
> And sorry for the noise. We had so many places which expected different
> behavior that I didn't noticed that two occasions in the fs code actually
> rely on the current behavior.
> 
> For your out of tree module you could make the code version independent by
> setting the vma back to the original file in case of an error. That should
> work with both behaviors in mmap_region.

Awesome, I'll give that a try, it may very well be a cleaner solution
either way.

And thank you for following up after your original patch and finding
the filesystems that mess around with those mappings. I'm sure it would
have taken me a while to figure out why file refcounts would go weird
for some people, especially because this only happens in the error path.

Jan